The field of the invention relates to recuperative or condensing furnaces. Generally, they are furnaces which transfer both sensible heat and heat of condensation from combustion products.
Nonrecuperative furnaces only transfer sensible heat from combustion products as they are cooled. Condensation does not occur during the heat transfer cycle since combustion products are exhausted at a temperature above their dew point. Heat transfer by nonrecuperative furnaces is, therefore, commonly referred to as a dry process.
On the other hand, recuperative furnaces not only transfer heat by the dry process of nonrecuperative furnaces but also simultaneously transfer heat of condensation. A separate recuperative heat exchanger is commonly used to transfer additional heat from combustion products after they have passed through a dry process. The recuperative heat exchanger cools combustion products sufficiently to condense thereby transferring both condensate heat and sensible heat. The additional heat transfer by the recuperative heat exchanger increases overall furnace efficiency to approximately 95%. Nonrecuperative furnances, on the other hand, are limited to 85%-88% efficiency.
Besides providing high efficiencies, the lower exhaust temperatures of recuperative furnaces enable the use of inexpensive exhaust venting such as, for example, pvc pipe rather than conventional chimneys. Further, low exhaust temperatures eliminate the draft hood associated with non-recuperative furnaces wherein heat is lost during the cooldown period at the end of each heating cycle.
Recuperative furnaces, however, are subject to corrosive attack of the recuperative heat exchanger by acidic condensate formed therein. In combusting natural gas, and to a greater extent fuel oil, a number of potentially acid forming gases are produced. Although these gases are typically noncondensable at the operating temperatures of the recuperative heat exchanger, they are absorbed by water vapor condensate thereby forming acids. For example, carbon dioxide forms carbonic acid, nitrogen dioxide forms nitric acid, hydrogen chloride forms hydrochloric acid, and hydrogen fluoride forms hydrofluoric acid. In addition, sulphur dioxide will condense within a recuperative heat exchanger thereby forming sulphurous acid. The acidity of the condensate is further increased when water condensate evaporates leaving behind concentrated acids which corrosively attack the heat exchanger.
Corrosive attack may also occur on heat exchange surface areas which are only exposed to combustion products that are above their dew point temperature. At the beginning of the heating cycle, incipient condensation may briefly form on initially cool surface areas. As these surfaces become heated during the heating cycle, the condensation evaporates and does not reoccur. Localized corrosion may therefore occur on these surfaces.
Several prior art approaches attempted to prevent corrosive attack. First, stainless steel components were used. However, it was found that chlorides are often present in our environment at levels which produce sufficient hydrochloric acid to corrode stainless steel. For example, chlorides are commonly found in laundry room areas which are often in close proximity to residential furnaces.
Second, hydrochloric acid resistant materials were proposed. A stainless steel molybedum alloy may be effective but is prohibitively expensive for residential heat exchangers. Although polymer or ceramic coatings may also be effective, they would increase thermal resistance and may also be subject to thermal shock. See U.S. Dept. of Energy, BNL51770, Condensing Heat Exchanger Systems for Residential/Commercial Furnaces and Boilers, Phase III, February 1984.
Third, U.S. Pat. No. 4,449,511 shows an external water flush system which is activated after each heating cycle to flush condensate from the coupling between the recuperator and burner. A potential corrosive problem may still exist in high chloride environments for corrosive attack on areas not flushed.